The present invention relates to an electronic circuit mounting structure, a power supply apparatus, a power supply system, and an electronic apparatus.
In the power supply apparatus and system, a power efficiency is a very important performance.
The power efficiency is a ratio of an output power to an input power of the power supply apparatus or power supply system, and an electricity that is not used as an output is lost mainly as heat within the system. Thus, if the power efficiency is low, a large amount of heat is produced in the power supply system, which means a large amount of electricity is lost and not used by electronic circuits or load. This in turn requires a structure or mechanism for cooling the power supply apparatus and system, increasing the cost and size. In addition, this cooling structure such as fan consumes electricity, further deteriorating the power efficiency of the power supply system including the fan. A high power efficiency on the other hand reduces the amount of heat generated, cost and size and enhances the performance of the power supply apparatus and system, contributing to energy saving and prevention of global warming.
The output voltage of the power supply apparatus and system for semiconductor circuits is falling year by year, so their power efficiency is deteriorating. As for semiconductor circuits, a microfabrication technology to achieve higher performance and integration lowers their dielectric strength. This necessarily reduces the power supply voltage of the semiconductor circuits. However, because of increased density integration and faster operation speed, the current consumption tends to increase. As a result, semiconductor circuits of the same kind tend to have an almost constant power consumption, a lower supply voltage and an increased current consumption. To meet this tendency of the semiconductor circuits, the power supply apparatus and system used for the semiconductor circuits have a reduced output voltage. In the power supply apparatus in general, if its output voltage is lowered with its output current kept constant, the amount of heat generated does not change greatly, so its power efficiency deteriorates. The power supply apparatus and system used for semiconductor circuits are thus deteriorating in its power efficiency year by year.
Various technological developments have been conducted to improve the power efficiency of the power supply apparatus and system or prevent its deterioration. All these technologies are intended to reduce a power supply loss or heat.
A technology of a switching power supply has improved the power efficiency significantly compared with a voltage dropper type power supply apparatus, a technology prior to the switching power supply. The technology of the switching power supply has been widely known through many publications. Most power supplies for semiconductor circuits are the switching power supply.
Technologies for preventing switching losses in the switching power supply, such as a zero voltage switching, have been developed. The switching losses are produced by charging or discharging of electrostatic capacitance of semiconductor circuits. However, storing energy in a coil and performing a switching in an appropriate order enables switching elements to be switched in a state where a voltage applied to the switching elements is zero, thus eliminating the switching losses. This is the zero voltage switching technique. Also a zero current switching technique has been developed.
To reduce losses in a rectification section of a switching power supply, a synchronous rectification circuit technology has been developed. The switching power supply is a circuitry that generates an alternating current and then rectifies it and has often used semiconductor diodes in the rectification section. Because the semiconductor diodes have a forward voltage drop of 0.5V to 1V regardless of a passing current, a heat generation may pose a problem in a power supply with a large current output. Replacing these rectification devices with field effect transistors (FETs) and having them perform a rectification operation in synchronism with an ac power is the synchronous rectification circuit technology. This technology reduces the voltage drop, which occurs between electrodes of the rectification devices, down to about 0.1V, significantly reducing the amount of heat generated, compared with the conventional diodes. Since the voltage drop is determined by an on-state resistance of FET, it is proportional to the flowing current. So, when an output current of the power supply is small, the amount of heat generated is small; and when the output current is high, the generated heat amount is large, a characteristic that is advantageous to the power supply. As a result, the synchronous rectification circuit technology, when combined with the above zero voltage switching technology, results in most of the losses in the switching power supply being composed of ohmic losses caused by resistive components such as on-state resistances of switching devices, winding resistances of transformers and wiring resistances, except for small operation power in control circuits.
Efforts to reduce resistive components include a technique that uses thick wiring members such as a metal plate. As disclosed in JP-A 2002-345245, this technology uses a metal plate or busbar in a large current wiring path or main current path. The switching power supply is an electronic circuit and thus often employs a construction in which components are mounted on a printed circuit board for connection. Since the wiring portion forming the printed circuit board is normally made of a thin copper foil, it has a large electric resistance and generates a large amount of heat when a large current is applied. This technology is intended to reduce the electric resistance and therefore the heat generated, by using a metal plate or busbar, a thick wiring member, in the main current path that carries a large electric current, thereby improving the power efficiency of the power supply system. According to the conventional technology disclosed in JP-A 2002-345245, a transformer and an upper part of components are connected through a thick metal plate to reduce electric resistance and realize an improved power efficiency.
The conventional technology will be explained by referring to FIG. 3. On a printed circuit board 18 are mounted a transformer 11 and semiconductor devices 15. Winding leads 12 of the transformer 11 are connected to metal electrode members 13, which in turn are connected to terminals 14 of the semiconductor devices 15. Other terminals of the semiconductor devices 15 are connected to a wiring pattern 16 which is connected to an output terminal 17. That is, the interconnects between the transformer 11 and the semiconductor devices 15 are realized by the metal electrode members 13, and the interconnects between the semiconductor devices 15 and the output terminal 17 are realized by the wiring pattern 16.
In this example, a printed circuit board with a copper wiring pattern 16 measuring 35 μm thick by 10 mm wide by 50 mm long normally has a wiring resistance of about 2.5 mΩ. If the metal plate electrode members 13 are 1 mm thick, 10 mm wide and 50 mm long, its resistance is about 0.1 mΩ. Thus, a total wiring resistance is about 2.6 mΩ. Let us consider a case where all wires are wiring patterns on the printed circuit board. In this case, since the wires connecting the transformer 11 and the semiconductor devices 15 are also a wiring pattern on the printed circuit board, there are two wiring patterns with a resistance of about 2.5 mΩ and the total resistance is about 5 mΩ. The conventional technology therefore replaces one of the wiring patterns with a metal plate electrode member to reduce the total resistance to 2.6 mΩ, about one-half the resistance of the ordinary construction. If the same current flows, the amount of heat produced is proportional to a resistance. When a current of 50 A flows, the wiring with 5 mΩ produces a heat of about 12 W while the wiring with 2.6 mΩ produces heat of about 6.5 W, which is about a half of the former.
As described above, this conventional technology has an effect of halving the electric resistance and the amount of heat generated in the wiring.
Since this construction has outside the printed circuit board 18, rather than on the board, the connections between the terminals 14 of the semiconductor devices 15 and the metal plate electrode members 13, these members cannot be assembled during the ordinary manufacturing process of printed circuit boards. However, the use of manual soldering allows for a technically easy assembling, though it is costlier than an automatic assembling.
As described above, research and development efforts have been made to improve the power efficiency of power supply apparatus systems.